The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The background of the invention is described below with reference to solar cells, as a specific example of the prior art.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back-side. It is well known that radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. The potential difference that exists at a p-n junction causes holes and electrons to move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metallized, i.e., provided with metal contacts which are electrically conductive.
During the formation of the solar cell, an Al paste is generally screen printed and dried on the back-side of the silicon wafer. The wafer is then fired at a temperature above the melting point of Al to form a Al—Si melt, subsequently, during the cooling phase, a epitaxially grown layer of silicon is formed that is doped with Al. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
Most electric power-generating solar cells currently used are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing from a metal paste.
An example of this method of production is described below in conjunction with FIG. 1. FIG. 1 shows a p-type silicon substrate, 10.
In FIG. 1(b), an n-type diffusion layer, 20 of the reverse conductivity type is formed by the thermal diffusion of phosphorus (P) or the like. Phosphorus oxychloride (POCl3) is commonly used as the gaseous phosphorus diffusion source; other liquid sources are phosphoric acid and the like. In the absence of any particular modification, the diffusion layer, 20 is formed over the entire surface of the silicon substrate, 10. This diffusion layer has a sheet resistivity on the order of several tens of ohms per square (Ω/□), and a thickness of about 0.3 to 0.5 μm. The p-n junction is formed where the concentration of the p-type dopant equals the concentration of the n-type dopant, conventional cells that have the p-n junction close to the sun side, have a junction depth between 0.05 and 0.5 um.
After formation of this diffusion layer, excess surface glass is removed from the rest of the surfaces by etching by an acid such as hydrofluoric acid. Next, a silicon nitride film, 30, is formed as an anti-reflection coating on the n-type diffusion layer, 20, to a thickness of between 0.05 and 0.1 um in the manner shown in FIG. 1(d) by a process, such as plasma chemical vapor deposition (CVD).
As shown in FIG. 1(e), a silver paste, 500, for the front electrode is screen printed then dried over the silicon nitride film, 30. In addition, a back-side silver or silver/aluminum paste, 70, and an aluminum paste, 60, are then screen printed (or some other application method) and successively dried on the back-side of the substrate. Normally, the back side silver or silver/aluminum is screen printed onto the silicon first as two parallel strips or as rectangles ready for soldering interconnection strings (presoldered copper ribbons), the aluminum is then printed in the bare areas with a slight overlap over the silver or silver/aluminum. In some cases, the silver or silver/aluminum is printed after the aluminum has been printed. Firing is then typically carried out in an infrared furnace at a temperature range of approximately 700 to 950° C. for a period of from several seconds to several tens of minutes. The front and back electrodes can be fired sequentially or co-fired.
Consequently, as shown in FIG. 1(f), molten aluminum from the paste dissolves the silicon during the firing process and then on cooling dopes the silicon that epitaxially grows from the silicon base, 10, forming a p+ layer, 40, containing a high concentration of aluminum dopant. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the solar cell.
The aluminum paste is transformed by firing from a dried state, 60, to an aluminum back electrode, 61. Prior art back-side aluminum pastes typically utilize aluminum particles of predominantly spherical shape derived from the atomization process where the particles are formed and wherein the particle sizes and shapes are not discriminated. The back-side silver or silver/aluminum paste, 70, is fired at the same time, becoming a silver or silver/aluminum back electrode, 71. During firing, the boundary between the back side aluminum and the back side silver or silver/aluminum assumes an alloy state, and is connected electrically as well. The aluminum electrode accounts for most areas of the back electrode, owing in part to the need to form a p+ layer, 40. Because soldering to an aluminum electrode is impossible, a silver or silver/aluminum back electrode is formed over portions of the back side (often as 2-6 mm wide busbars) as an electrode for interconnecting solar cells by means of pre-soldered copper ribbon or the like. In addition, the front electrode-forming silver paste, 500, sinters and penetrates through the silicon nitride film, 30, during firing, and is thereby able to electrically contact the n-type layer, 20. This type of process is generally called “firing through.” This fired through state is apparent in layer 501 of FIG. 1(f).
Additionally, while conventional solar cells provide a working design, there is still a need to provide higher efficiency devices. There is also a need to provide a method of forming such a device at lower temperatures than the prior art. Such a method would allow for co-firing over a wider range of temperatures so as to provide manufacturers with increased flexibility and would enable compensation of thermal work for thicker and thinner wafer sources. The present invention provides such a device and method for its formation.
Furthermore, there is an on-going effort to provide compositions which are Pb-free while at the same time maintaining electrical performance and other relevant properties of the device. The present inventors create novel aluminum comprising composition(s) and semiconductor devices that simultaneously provide such a Pb-free system while still maintaining electrical performance and novel compositions that provide superior electrical performance. Furthermore, the composition(s) of the present invention lead to reduced bowing in some embodiments of the invention.